The present invention relates to a control system for driving a DC motor which operates a shifting mechanism.
It is known to use electric motors to actuate shifting mechanisms. For example, there are shifting mechanisms for manual transmissions which are actuated automatically by electric motors. In order to prevent overloading of the shifting mechanism, the shifting force during the shifting process must be restricted. The shifting force is usually limited in such shifting mechanisms by incorporating elastic elements that absorb the excess forces into the power train and/or by using, for example, force or position sensors for limiting the electric current applied to the motor.
A method and device for controlling the shifting force in an automatic mechanical transmission is disclosed in U.S. Pat. No. 5,481,170. In this system an electric motor is controlled via pulse-width modulation by a control unit. The control unit controls or regulates the duty cycle of a pulse-width modulated (PWM) voltage signal that is altered and adapted as a function of predetermined limit values. For example, the current is monitored during the shifting process and compared with a permissible target current value in order to avoid excessive current fluctuations. With this system it is possible to limit and monitor the torque of a DC motor which generates the shifting force so as to prevent mechanical overload of components of the shifting mechanism. However, the method described in U.S. Pat. No. 5,481,170 requires that the current be sensed so that the sensed current signal can be used in the control process. This increases the cost and complexity of the system.
EP 0 126 988 A1 discloses a control circuit for regulating the speed of a DC motor. The control circuit includes a speed regulating component that generates a pulse-width modulated control signal as a function of a speed sensor, i.e., as a function of the motor speed, and a predetermined nominal speed. This control signal is used to control a transistor switching bridge circuit which drives the DC motor. The speed sensor generates a signal that is proportional to the speed of the DC motor. This signal is used for changing the duty cycle of a pulse-width modulated control signal for the transistor switching bridge of the DC motor. The DC motor is alternately supplied with positive and negative current in accordance with the duty cycle of the control signal so that the DC motor is actively alternately accelerated and decelerated. The active deceleration, i.e., supplying the DC motor with current opposite to the rotational movement, should also make it possible to control the speed and rapidly to decelerate the motor at low speeds. The transistor switching bridge circuit has a conventional design. Four transistors are arranged between the terminals of a voltage supply and the DC motor in the form of a bridge circuit. The transistors are switched in such a way that the two sets of the diametrically opposite transistors of the bridge circuit are respectively switched to the conductive and non-conductive state. Thus, two current paths for supplying the DC motor are realized, where the current flows through these current paths alternately and in opposite directions. These current paths are controlled by two control transistors, the collectors of which are respectively connected to the bases of two transistors of the bridge circuit. The pulse-width modulated control signal generated by the speed-regulating component is applied to the bases of the control transistors. If one of the two control transistors is switched to the conductive state, the current flows through the DC motor along one of the current paths of the transistor switching bridge. The motor is alternately driven in one direction of rotation and in the other direction of rotation due to the pulse-width modulated, alternate switching of the control transistors. Depending on the pulse-width ratio, the drive of the DC motor predominates in one direction of rotation so that the speed of the motor can be controlled.
The control circuit of EP 0 126 988 A1 has two controllable current paths that are alternately triggered in order to operate the DC motor. However, the regulation of the speed is complex and costly, since additional components are required, such as a speed sensor and a logic unit for the signal logic operation.
The above described methods and devices are unable to limit both the torque and the speed of a DC motor.